Americans respect marriage, not only as a crucial institution of civil society but the fundamental building block of all human civilization. This is why 41 states and the federal government affirm that marriage is between a man and a woman.

But these laws are on the line. Next week, the U.S. Supreme Court will hear oral arguments in cases that challenge the constitutionality of the federal Defense of Marriage Act (DOMA) and California’s Proposition 8. The court should uphold these laws and respect the constitutional authority of citizens and their elected officials to make marriage policy.

All Americans stand equally before the law and have their civil rights equally protected—rights to free speech, religious liberty, free association and every other traditional civil liberty. The question is whether a new “civil right”—the right to have the government and private citizens recognize one’s same-sex sexual partnership as a marriage—ought to be created, redefining marriage in the process.

The government isn’t in the business of affirming our loves. Rather it leaves consenting adults free to live and love as they choose.

And contrary to what some say, there is no ban on same-sex marriage. Nothing about it is illegal. In all 50 states, two people of the same sex may choose to live together, choose to join a religious community that blesses their relationship, and choose a workplace offering joint benefits.

What’s at issue is whether the government will recognize such relationships as marriages—and then force every citizen, house of worship and business to do so as well. At issue is whether policy will coerce and compel others to recognize and affirm same-sex relationships as marriages.

Being created equal doesn’t entail or require redefining marriage. Every marriage policy draws lines, leaving out some types of relationships. It’s true that “marriage equality” makes a good slogan for activists and politicians, but true equality forbids arbitrary line-drawing. It requires that the state get marriage right. To do so we need to answer two questions:

What is marriage?

Why does it matter for policy?

Reflecting on these questions reveals why there’s nothing “equal” about redefining marriage to eliminate sexual complementarity, which has been the foundation of marriage throughout history.

Marriage exists to bring a man and a woman together as husband and wife to be father and mother to any children their union produces. Marriage is based on the biological fact that reproduction depends on a man and a woman, and on the social reality that children need a mother and a father. And as ample social science has shown, children tend to do best when reared by their mother and father.

Marriage has public purposes that transcend its private purposes. Government recognizes marriage because it is an institution that benefits the public good. Marriage is society’s least restrictive means to ensure the well-being of future citizens. State recognition of marriage protects children by incentivizing adults to commit permanently and exclusively to each other and their children.

As with other public policy issues, religious voices on marriage should be welcomed in the public square. Yet as my co-authors and I argue in our new book “What Is Marriage? Man and Woman: A Defense,” one need not appeal to distinctively religious arguments at all to understand why marriage—as a natural institution—is the union of man and woman.

This understanding has been shared by the Jewish, Christian and Muslim traditions; by ancient Greek and Roman thinkers untouched by these religions; and by various Enlightenment philosophers. It is affirmed by both common and civil law, and by ancient Greek and Roman law.

And far from having been intended to exclude same-sex relationships, marriage as we always have known it arose in many places over several centuries, when same-sex marriage was nowhere on the radar. Indeed, marriage arose in cultures that had no concept of sexual orientation, and in some that fully accepted homoeroticism and even took it for granted.

In recent years marriage has been weakened by a revisionist view that is more about adults’ desires than children’s needs. Americans increasingly are tempted to think that marriage is simply whatever sort of relationship consenting adults—be they two or 10 in number—want it to be: sexual or platonic, sexually exclusive or open, temporary or permanent.

Redefining marriage to exclude the idea that it is fundamentally related to the union of a man and woman would make emotional intensity the only thing left to set marriage apart from other kinds of relationships. Redefining marriage would put a new principle into law—that marriage is whatever emotional bond the government says it is.

No principled reason could be offered for why an emotional union should be permanent. Or limited to two persons. Or exclusive (as opposed to “open”). Couples might live out these norms where temperament or taste motivated them, but no reason of principle would compel them.

Marriage simply can’t do the work that society needs it to do for generations to come if we weaken the norms further. All of us who care about a thriving civil society, with institutions capable of limiting the state and its power, should be alarmed.

Support for marriage between a man and a woman is no excuse for animus against those with same-sex attractions, or for ignoring the needs of individuals who may never marry, for whatever reason. They are no less worthy than others of concern and respect.

And we can craft policy that benefits all Americans without redefining marriage at all. Sometimes it’s as simple as repealing a bad tax.

Concern for the common good requires protecting and strengthening the marriage culture, by promoting the truth about marriage. It requires getting marriage right.

While respecting everyone’s liberty, government rightly recognizes, protects and promotes marriage between a man and a woman as the ideal institution for procreative love, childbearing and childrearing. Recognizing that we are all created equal doesn’t challenge this historic understanding.

-Ryan T. Anderson is the William E. Simon Fellow in Religion and a Free Society at The Heritage Foundation and editor of the Witherspoon Institute’s online journal Public Discourse. He is the author with Sherif Girgis and Robert P. George of “What Is Marriage? Man and Woman: A Defense” (Encounter, 2012).